Generally speaking, optical wavelength multiplexing and demultiplexing have been accomplished in the past by using an interconnection apparatus having a plurality of closely spaced input waveguides communicating with the input of a star coupler. The output of the star coupler communicates with an optical grating comprising a series of optical waveguides, each of the waveguides differing in length with respect to its nearest neighbor by a predetermined amount. The grating is connected to the input of a second star coupler, the outputs of which form the outputs of the switching, multiplexing, and demultiplexing apparatus. Examples of such interconnection apparatuses are disclosed in U.S. Pat. Nos. 5,002,350, 5,136,671 and 5,412,744.
The geometry of such an interconnection apparatus may be such that a plurality of separate and distinct wavelengths each launched into a separate and distinct input port of the apparatus will all combine and appear on a predetermined one of the output ports. In this manner, the apparatus performs a multiplexing function. The same apparatus may also perform a demultiplexing function. In this situation, an input wavelength is separated from the others and directed to a predetermined one of the output ports of the apparatus. An appropriate selection of input wavelength also permits switching between any selected input port to any selected output port. Accordingly, these devices are generally referred to as frequency routing devices and more specifically wavelength division multiplexers (WDM).
Ideally, the operation of these WGR and WDM optical devices should by predictable and consistent over a wide range of environmental conditions. Unfortunately however, in practice, the operational performance of such devices is significantly affected by variations in the temperature of the environment surrounding the device.
More specifically, the wavelength spectrum of existing WGR designs shifts with temperature (T) for at least two reasons. First, where n represents the refractive index of the waveguide material, dn/dT.noteq.0 and secondly, the thermal expansion, i.e. dL/dT, where L represents length, likewise does not equal zero.
To date, some of the techniques used to create optical devices that are less sensitive to temperature changes have included using a heater with a temperature controller to stabilize the wavelength spectrum of the WGR. Unfortunately, such a design is expensive and impractical in applications where electrical power is not readily available. In addition, the semiconductor art has demonstrated a temperature insensitive semiconductor WGR that includes a waveguide region with different dn/dT.
However, even in light of the technical advances mentioned above, there remains a definite need for a practical design and method for making optical waveguide filtering devices temperature independent. This is particularly true for compensating waveguide grating routers, which are, at present, the multiplexers of choice for dense WDM systems. Furthermore, given that silicon optical bench routers are now components of various Next Generation Lightwave Networks (NGLN) and are planned for use in Fiber-to-the-Home (FTTH) access networks, temperature-compensating optical devices like WGRs are highly desirable for these networks.